Rac GTPases are important signal transducers in many aspects of neural development. Past studies of Rac GTPases mainly used over-expression of constitutive-active or dominant- negative mutants of Rac1. This approach cannot distinguish between Rac1 and its closely related isoform Rac3, and may interfere with the functions of other Rho GTPases such as Cdc42. To overcome these obstacles, we have employed a conditional gene-targeting strategy using Foxg1-Cre mice to delete the floxed Rac1 or Cdc42 in telencephalic progenitors. These studies have revealed unique as well as novel functions of Rac1 (see Preliminary Results). Based on these results, we hypothesize that Rac1 has important functions in the proliferation and differentiation of ventral telencephalic progenitors, and it transduces Netrin/DCC-mediated signal for the midline commissural tract formation. Moreover, Rac1 may have critical, but overlapping functions with Rac3 in axonal growth and spine morphogenesis. We will test our hypotheses in three specific aims. In Aim 1 we will use conditional Rac1-null, Rac3-null, and Rac1/Rac3 compound mutants to examine the development of the forebrain. In addition, we will use neurosphere cultures to study the specific functions of Rac in neural progenitors and the involved mechanisms. In Aim 2 we will use a series of explant and cell culture assays to examine whether Rac1 is needed for netrin-induced signal transduction and growth cone functions. In Aim 3 we will also use the Rac1/Rac3 compound mutant embryos and postnatal deletion of Rac1 in the Rac3-null genetic background to determine whether Rac1 and Rac3 collectively regulate axonal outgrowth and the dendritic spine morphogenesis. In summary, this project will address three important aspects of brain development in which Rac GTPases either have novel functions as indicated by preliminary results (Aim 1 &2) or are expected to play a critical role and warrant special attention (Aim 3). Our goal is to bridge functional specificity analysis to mechanistic understanding of these critical signal transducers. PUBLIC HEALTH REVELANCE: Rac GTPases are important signal transducers in many aspects of neural development, but their physiologic functions could be masked in previous studies by over-expression of dominant mutants. Using conditional gene-targeting, the principal investigator has uncovered novel functions Rac GTPases in neural progenitors and the Netrin-mediated axon guidance. The present project will determine the mechanism of these important functions, and further explore the roles of Rac GTPases in axonal outgrowth and the dendritic spine morphogenesis.